The invention disclosed relates to an improved transceiver and method for transceiver operation in cable television or similar broadband cable communications systems.
Cable-television systems have, in recent years, moved beyond merely providing television signals by cable to system subscribers in their homes. The subscriber has been given a voice in many systems. These advantaged subscribers have transceivers which allow them to transmit signals to the controlling or headend facility of the system. Among the many services which subscribers use through such signals are the purchase of extra-cost special programs and home shopping on television catalogue services.
A subscriber sends a signal to the headend facility through a keyboard or other generating means connected to his transceiver. His entry to the keyboard is encoded in a digital format and becomes, potentially with the address of his station and other enabling data, a data packet for transmission to the headend facility. The data packet is used to modulate a radio frequency carrier wave and is transmitted to the headend facility through the cable plant of the cable television system.
The cable plant is the communications medium of a cable television system or local area network. Typically, it comprises a broadband transmission cable over which signals are transmitted from the headend facility to home transceivers (outbound signals) as well sa from home transceivers to the headend facility (inbound signals). Broadband coaxial cables are advantageously used in this application because, inter alia, they support a large frequency bandwidth and provide signal shielding at a moderate cost in comparison to other media. The wide frequency bandwidth permits the definition of a substantial number of channels on the cable allowing simultaneous inbound and outbound signals. Each signal occupies a particular channel, i.e. frequency bandwidth, on the cable.
A given channel can effectively support only one signal at a time. If more than one signal appears simultaneously on a channel, one signal source will dominate the detectors listening to the channel, excluding all weaker signal sources while it is transmitting, or, where the signal strength of individual transmitters is more or less equal, the signals will interfere with one another and the intelligence comprehended by each signal will be confused with that of other signals. In either case, access by some or all transceivers is lost. Were channel availability on a coaxial cable unlimited, such matters would be of no consequence. Each subscriber could be given a dedicated channel on the cable for access to the headend facility. Of course, this is not the case. The number of channels available is always far less than the number of subscribers desiring to use the system.
However, subscriber-initiated inbound signals are typically of short duration. Such a signal, in the equipment with which the invention is associated in cable-television systems, lasts only 1.6 microseconds. Nor will signals typically be initiated by a subscriber with great frequency.
Accordingly, the system designer should only need to assign one or, at most, a handful of channels to accomodate all inbound signals generated by subscribers. He may do so in anticipation of the short duration and relative paucity of signals from any given subscriber. A single channel may potentially be used for thousands of subscribers without interference between signals becoming apparent to the subscriber.
The system designer's ability to rely on a single channel to accomodate all subscribers will flow from reliable operation of the home transceivers coupled to the cable plant. Were a single home transceiver to fail and latch into its signal generating mode, the channel assigned to home transceivers for inbound signals could be lost. Potentially, a few transceivers could overpower the latched signal generator, but power limitation considerations may make even this inelegant, partial response unavailable. Failure of individual transceivers is impossible to prevent where low cost is a critical consideration. However, reliable system operation does not necessarily mean fail-safe operation of individual tranceivers. It can include a broader range of possibilities based on predictable operation of those tranceivers. It is desirable to provide an apparatus and method by which a failed transceiver simply disables itself. In this manner, a transmitter which has failed will disappear to the communications system. Initiation of measures to disable a transmitter may be taken upon indication that a transmitter has transmitted for a period of time exceeding in length the maximum time period allowed for transmission of a data packet.